In general, in a power plant or the like, a denitration apparatus is used for treating nitrogen oxide in combustion flue gas generated from a combustion facility. The combustion facility includes a boiler for coal combustion, gas combustion, oil combustion or the like, and a gas turbine and the like. The denitration apparatus adds, on its upstream side, a reducing agent such as ammonia and an ammonia compound to the flue gas, and reacts the reducing agent and nitrogen oxide in the flue gas on a denitration catalyst provided in the denitration apparatus to perform reduction treatment into nitrogen. Basically, the reducing agent is fed as gas, or is directly sprayed into the flue gas as a solution. Since even in the case of the solution spraying, it is heated and vaporized by the high-temperature flue gas, it is eventually to be added in a gas form.
Now, the amount of the flue gas as the denitration treatment target reaches 3 million m3N/h, for example, in the case of a 1000 MW-class power generation facility. On the contrary, that of the reducing agent is 9000 m3N/h even including dilution air and the like. In other words, since the flue gas amount is approximately 300 times as much as that of the reducing agent gas, in order to enhance denitration efficiency, a very small amount of reducing agent gas is needed to be uniformly dispersed in a large amount of flue gas. Emission regulation of nitrogen oxide (NOx) from the system has particularly tended to be enhanced, and, for example, 90% or more of denitration rate is requested. Meanwhile, the concentration of slip ammonia due to flowing-out of unreacted ammonia, which is the reducing agent, from the denitration apparatus is regulated to be not more than several ppm. In order to satisfy such regulation, it is important to control a molar ratio of ammonia (NH3) relative to nitrogen oxide (NOx) or the upstream of the denitration catalyst not to exceed 1.
For example, Patent Literature 1 proposes to independently control ammonia injection amounts for respective plurality of regions by dividing a flow channel cross-section of a flue gas duct into the regions and disposing a plurality of ammonia injection nozzles for the respective regions. According to this, the NOx concentration or the slip ammonia concentration on the flow channel cross-section of a denitration catalyst outlet can be actually measured to perform fine adjustment by performing feedback control on the ammonia injection amounts for each of the regions.
Moreover, it is generally performed to use a gas mixer as disclosed in Patent Literature 2. This is installed in a flue gas duct between an ammonia injection nozzle and a denitration catalyst, and an effect of mixing the flue gas and the ammonia gas is expected.